The present invention relates to an output matching apparatus of a microwave Doherty amplifier; and, more particularly, to a full output matching apparatus of a microwave Doherty amplifier capable of obtaining a full output matching at a super high frequency band by performing an additional phase tuning process after a conventional microwave output matching of the Doherty amplifier.
As is well known, a microwave Doherty amplifier is a high efficiency amplifier capable of performing only a genuine resistive output matching process. The Doherty amplifier generally uses two amplifier paths, a carrier amplifier and a peaking amplifier, and controls a load line impedance of the carrier amplifier by varying, depending on a power level, an amount of current provided from the peaking amplifier to a load. To perform such a function of high efficiency, the Doherty amplifier employs a technique where the carrier amplifier and the peaking amplifier are connected in parallel to each other by a quarter wave transformer (xcex/4 line).
The microwave Doherty amplifier was used in earlier days as an AM transmitter of a broad casting apparatus using a high-power LF/MF vacuum tube. Then, various suggestions have been made to apply the Doherty amplifier to a solid-state high-power transmitter. FIG. 1 shows one of such suggestions.
The microwave Doherty amplifier in FIG. 1 uses quarter wave transformers 31 to 39 for an output matching of a carrier amplifier 10 and a peaking amplifier 20. In this output matching system, only a resistive matching is possible.
Referring to FIG. 2, there is provided a microwave Doherty amplifier using another conventional output matching system. Shunt reactive load impedances 41 and 42 are connected to an output section of the carrier amplifier 10 and the peaking amplifier 20, respectively. Further, a microwave Doherty network 50 is coupled to the output sections of the carrier amplifier 10 and the peaking amplifier 20. However, this conventional output matching system also has a drawback in that an output matching may not be obtained for both real and imaginary parts and the shunt reactive load impedances 41 and 42 may deteriorate the efficiency of the Doherty amplifier.
Accordingly, there has been intensified a demand for a full output matching apparatus capable of obtaining a full output matching for both real and imaginary parts while preserving a highly improved efficiency of the solid-state microwave Doherty amplifier.
It is, therefore, an object of the present invention to provide a full output matching apparatus of a Doherty amplifier which uses a characteristic impedance line added after an output matching of a microwave Doherty amplifier as a phase tuning component such that a matching state can be maintained without being changed at a high power level but can be adjusted depending on a phase variation at a low power level to obtain efficiency enhancements and optimum linearity.
In accordance with the present invention, there is provided a microwave Doherty amplifier for inducing a Doherty operation, wherein quarter wave transformers are connected to a final output of a carrier amplifier and a peaking amplifier coupled in parallel to each other, the Doherty amplifier comprising: a load matching circuit connected to output terminals of the carrier amplifier and the peaking amplifier so as to obtain a microwave output matching; and a phase tuning component located behind the load matching circuit.